1. Technical Field
The present invention relates to a driver device for driving light emitting elements.
More particularly, the present invention relates to a driver device for driving light emitting elements (LEDs) in electronic systems.
2. Description of the Related Art
Extending battery life and reducing power consumption are major challenges facing electronic system designers.
This particularly applies both to portable electronic systems, such as cell phones, digital cameras, notebooks, and to those electronic systems that operate even when they are disconnected from the mains, using a battery and/or solar cell supply, as well as to any other electronic system in which energy saving is a primary design requirement.
Therefore, the designers of these electronic systems have to face both the desire of reducing the size of the electronic devices used in such systems (and the battery size) and the desire of reducing power consumption for operation thereof, while extending battery life.
Light emitting diodes (LEDs) are commonly used in such electronic systems.
These LEDs find application, for instance, in LED panels for home appliances, LED back-light LCD panels of mobile phones, LCD signboards, etc.
At present, LED panels are designed with SIPO (Serial Input Parallel Output) driver devices which are electrically coupled together in a so-called daisy-chain configuration.
Particularly, the LEDs of a panel may be driven by these driver devices, each of which can supply a constant current to such LEDs.
Referring to FIG. 1, which shows an approach used for an LED display driving circuit, a LED matrix 1 is shown, which is driven by a driver system 2 having a plurality of driver devices 2A, . . . , 2E.
The driver devices 2A, . . . , 2E are coupled together in a “daisy chain configuration”, which means, for example, that the driver device 2D has at its input the output data of the driver device 2C and that this driver device 2D has data at its output for the driver device 2E.
It is further noted that the LED matrix 1 receives, for instance, a 4 V supply voltage, such voltage being generated by a DC/DC switch-mode power supply 3 from a power source that can provide a voltage ranging from 5 V to 35 V.
Still referring to FIG. 1, the driver system 2 is controlled by a microcontroller 4 which can receive control data 5 from an interface panel 6.
The driver system 2 and the microcontroller 4 receive, for instance, a 3.3 V supply voltage, such voltage being generated by a DC/DC converter 7 from a power source that can provide a voltage ranging from 5 V to 35 V.
Particularly, in the specific circuit diagram of FIG. 1, the driver system 2 comprises five driver devices 2A, . . . , 2E, each having a control logic and an analog output stage for driving the LEDs of the LED matrix 1 by a constant current, and being further characterized by having an N-bit SIPO shift register, wherein data is serially loaded into such register but appears to be simultaneously collected, at a predetermined time from all outputs in parallel.
For instance, the driver device 2A is designed to serially receive control data 5 from the microcontroller 4 and the output stage of such driver device 2A has N driver channels, each being able to drive one or more LEDs of the LED matrix 1.
In the specific illustration of FIG. 1, the LED matrix 1 is composed of eighty LEDs and each driver device 2A, . . . , 2E can drive sixteen LEDs.
In other words, each driver device 2A, . . . , 2E has an output stage with sixteen separate driver channels, each of such driver channels being adapted to drive one LED of the LED matrix 1.
The current flowing through each driver channel has to be a constant current and for such driver devices 2A, . . . , 2E to provide such constant current, there is a continuous consumption of a certain amount of current that flows through the device and/or through a dedicated pin.
Particularly, an external resistor 2F is used with the driver device, which resistor is coupled to a current controller 2G to provide constant supply current to the LEDs of the LED matrix 1.
While the circuit diagram of FIG. 1 provides undisputable advantages, such as the possibility of integrating the driver devices 2A, . . . , 2E into a single integrated circuit, it still suffers from certain drawbacks, e.g., that of consuming power even when the driver system 2 does not receive at its input the data 5 required for controlling the LEDs of the matrix from the microcontroller 4.
In other words, the driver system 2 dissipates energy even when there is no need for all the LEDs of the matrix 1 to be on.
In fact, these circuit diagrams often have less than 50% of the driver devices on at the same time.
For example, in a video display LED application that can use even hundreds of thousands of devices, current consumption may be on the order of a few tens of amperes.
As an alternative, driver systems have been provided which are implemented through the use of discrete devices.
While these driver systems allow some savings in dissipated power, they still suffer from the drawback of higher costs and greater complexity as compared with integrated configurations.